1. Field of the Invention
The present invention relates to sensors for detecting an oriented physical variable, and relates more particularly to magnetic field sensors.
The present invention more particularly relates to pendular sensors, i.e., sensors including a pendular system on which at least a detector is disposed, an electronic card, and electric wires to connect the detector to the electronic card.
2. Discussion of the Related Art
The function of the pendular system is to orientate the detector. More particularly, a pendular system having two degrees of liberty (a roll axis and a pitch axis) maintains the detector in a constant horizontal plane. A pendular system having one degree of liberty (only a roll axis) orientates the detector along a constant direction with respect to the sensor.
The main drawback of such sensors originates from the presence of electric wires, which are required to connect the detector, which is mobile, and the sensor electronic card, which is fixed and cannot be mounted on the pendular system. Indeed, the electric wires impair the pendular mechanism and limit the sensor's performances, as will be better understood with relation to the example illustrated in FIG. 1.
FIG. 1 represents an exploded view of a conventional pendular magnetometer 20 having two degrees of liberty, for detecting the two horizontal components of the earth's magnetic field. This detector can be used for the determination of a course, or an orientation, or simply the measurement of a local magnetic field. This magnetometer is, for example used in the field of sub-marine petrol research, in which the presence of oil fields is detected by measuring the echoes caused by an explosion. The measurement is achieved with hydrophones fixed to a sub-marine cable pulled by a ship. Since the orientation of the hydrophones should be known with a high accuracy, and since the cable may be twisted or corkscrewed, several magnetometers of this type are disposed at regular intervals along the cable.
Magnetometer 20 is provided with two magnetic field detectors 1, 2, that are perpendicularly disposed, each detector being assigned to the measurement of one of the two components of the earth's magnetic field. Detectors 1, 2 are mounted on a pendular support 3 articulated with a Cardan system, and is horizontally maintained by a return mass 4. Support 3 is integral of a first axis 6-1 mounted through roll-bearings (with balls, needles, or other suitable means) carried by a frame 7. Frame 7 is in turn integral with a second axis 6-2 guided by roll-bearings carried by the casing 8-1 of the sensor. Hereinafter, the axis carried by the sensor casing 8, in this case axis 6-2, will be called roll axis, and the intermediate axis 6-1 carried by frame 7 will be called pitch axis. Both axes are oriented along perpendicular direction AA' and BB'. The sensor is enclosed in a water-proof casing 8 filled with a damping lubrication oil. Frame 8-1 constitutes the upper portion of the casing 8 whose lower portion is a bowl shaped portion 8-2, accommodating the whole pendular system. The two detectors 1, 2 are connected to an external electronic card 10 fixed on the top of frame 8-1, through conductive wires 5 crossing the casing with water-proof means. Inside the casing, the wires must have a sufficient length to not prevent the pendular system 3 from reaching its horizontal position when the casing is tilted. In addition, to obtain a high accuracy in the horizontal positioning, the rigidity and weight of wires 5 should be as low as possible. Thus, very thin breaded copper wires, covered with a polyester synthetic fiber are conventionally used.
However, these precautions do not avoid the main drawback of the device, which is the highly limited angular movement of the pendular system 3, with respect to the casing, due to the provision of wires. Theoretically, the maximum possible inclination of the sensor with respect to the longitudinal axis is .+-.180.degree. with respect to the vertical position. In practice, the maximum inclination values are substantially lower, because in the long run maximum inclination causes wires to be excessively worn out and also reduces the reliability of the sensor. That is why the sensor 20 represented in FIG. 1 is designed so that the pendular mass 4 abuts against the bowl 8-2 in case of a high inclination of the sensor, which corresponds to a maximum pitch rotation of 120.degree. (i.e., .+-.60.degree.). For the same reason, the frame 7 abuts against the casing 8-1, for a maximum roll rotation of 120.degree..
In many applications, the limitation of the pitch axis is tolerable, but the roll limitation is an impairing constraint. In the above application, for example, it is desired that the sensor has an unlimited rotation about the roll axis, because the cable bearing the magnetometers can be twisted and the sensors can achieve several 360.degree. rotations. It will be understood that the conventional sensor is not adapted to such operation conditions, because the pendular system can abut and no more be able to reach the horizontal position. In other applications, a rotation higher than 120.degree., unlimited or not, will be desired.